Author response: Competition between Tropomyosin, Fimbrin, and ADF/Cofilin drives their sorting to distinct actin filament networks

Christensen, Jenna R; Hocky, Glen M.; Homa, Kaitlin E; Morganthaler, Alisha N; Hitchcock‐DeGregori, Sarah E.; Voth, Gregory A.; Kovar, David R.

doi:10.7554/elife.23152.024

Cited by 2 publications

(1 citation statement)

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“…This observation predicts that the generation of tropomyosin-free actin filaments would, of necessity, require the active inhibition of tropomyosin binding to filaments. Some actin-binding proteins known to compete with tropomyosin include cofilin, fimbrin, and a-actinin [9,33] as well as Arp2/3-branched actin networks, which are incompatible with tropomyosin binding [34]. Conversely, mediators of tropomyosin recruitment like formins [6,35] and a subset of myosins [5,36,37] are expected to favor the binding to actin of specific tropomyosin isoforms.…”

Section: Tropomyosin Quantificationmentioning

confidence: 99%

Co-polymers of Actin and Tropomyosin Account for a Major Fraction of the Human Actin Cytoskeleton

et al. 2018

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Tropomyosin proteins form stable coiled-coil dimers that polymerize along the α-helical groove of actin filaments [1]. The actin cytoskeleton consists of both co-polymers of actin and tropomyosin and polymers of tropomyosin-free actin [2]. The fundamental distinction between these two types of filaments is that tropomyosin determines the functional capability of actin filaments in an isoform-dependent manner [3-9]. However, it is unknown what portion of actin filaments are associated with tropomyosin. To address this deficit, we have measured the relative distribution between these two filament populations by quantifying tropomyosin and actin levels in a variety of human cell types, including bone (U2OS); breast epithelial (MCF-10A); transformed breast epithelial (MCF-7); and primary (BJpar), immortalized (BJeH), and Ras-transformed (BJeLR) BJ fibroblasts [10]. Our measurements of tropomyosin and actin predict the saturation of the actin cytoskeleton, implying that tropomyosin binding must be inhibited in order to generate tropomyosin-free actin filaments. We find the majority of actin filaments to be associated with tropomyosin in four of the six cell lines tested and the portion of actin filaments associated with tropomyosin to decrease with transformation. We also discover that high-molecular-weight (HMW), unlike low-molecular-weight (LMW), tropomyosin isoforms are primarily co-polymerized with actin in untransformed cells. This differential partitioning of tropomyosins is not due to a lack of N-terminal acetylation of LMW tropomyosins, but it is, in part, explained by the susceptibility of soluble HMW tropomyosins to proteasomal degradation. We conclude that actin-tropomyosin co-polymers make up a major fraction of the human actin cytoskeleton.

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Section: Tropomyosin Quantificationmentioning

confidence: 99%

Co-polymers of Actin and Tropomyosin Account for a Major Fraction of the Human Actin Cytoskeleton

et al. 2018

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Actin filament oxidation by MICAL1 suppresses protections from cofilin-induced disassembly

Wioland

Frémont

Guichard

et al. 2020

Preprint

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Proteins of the ADF/cofilin family play a central role in the disassembly of actin filaments, and their activity must be tightly regulated in cells. Recently, the oxidation of actin filaments by the enzyme MICAL1 was found to amplify the severing action of cofilin through unclear mechanisms. Two essential factors normally prevent filament disassembly: the inactivation of cofilin by phosphorylation, and the protection of filaments by tropomyosins, but whether actin oxidation might interfere with these safeguard mechanisms is unknown. Using single filament experiments in vitro, we found that actin filament oxidation by MICAL1 increases, by several orders of magnitude, both cofilin binding and severing rates, explaining the dramatic synergy between oxidation and cofilin for filament disassembly. Remarkably, we found that actin oxidation bypasses the need for cofilin activation by dephosphorylation. Indeed, non-activated, phosphomimetic S3D-cofilin binds and severs oxidized actin filaments rapidly, in conditions where non-oxidized filaments are unaffected. Finally, tropomyosin Tpm1.8 loses its ability to protect filaments from cofilin severing activity when actin is oxidized by MICAL1. Together, our results show that MICAL1-induced oxidation of actin filaments suppresses their physiological protection from the action of cofilin. We propose that in cells, direct post-translational modification of actin filaments by oxidation is a way to trigger their severing, in spite of being decorated by tropomyosin, and without requiring the activation of cofilin.

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Author response: Competition between Tropomyosin, Fimbrin, and ADF/Cofilin drives their sorting to distinct actin filament networks

Cited by 2 publications

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Co-polymers of Actin and Tropomyosin Account for a Major Fraction of the Human Actin Cytoskeleton

Co-polymers of Actin and Tropomyosin Account for a Major Fraction of the Human Actin Cytoskeleton

Actin filament oxidation by MICAL1 suppresses protections from cofilin-induced disassembly

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